1. Field of the Invention
The present invention relates to methods of refining biomass. More specifically, the present invention relates to methods of disrupting the cellular structure of biomass and conditions to hydrolyze biomass while conserving and reducing process energy and minimizing equipment required while creating highly concentrated products in short timeframes.
2. Description of the Related Art
At present, the United States produces ethanol from starch-containing corn seed using amylase enzymes to dissolve the starch to fermentable sugars, which are then fermented to ethanol using yeast. In general, while the starch in the corn seed is used in the production of ethanol, the remainder of the corn plant biomass from which the seed is extracted, i.e., the leaves, cobs and stalks, is not presently used to produce ethanol because of the lack of a practical process associated with dissolving the non-starch corn components to fermentable sugars. Thus, the ligno-cellulosic components of corn biomass represent a tremendous source of untapped energy that remains unused because of the difficulty and cost of converting it to fermentable sugars. However, from a broader biomass perspective, corn stalks and cobs represent only a small portion of biomass feedstock potential world wide. For example, the volume and cost of tropical grasses grown in poor countries could provide sugars sufficient to produce tens of billions of gallons of biofuels if a practical process existed.
Currently, there are four main technologies being researched to convert cellulose to fermentable sugars, with none of them enjoying large scale commercialization. These are: concentrated acid hydrolysis, dilute acid hydrolysis, biomass gasification and fermentation, and enzymatic hydrolysis.
Concentrated acid easily dissolves biomass. Separation of acid from sugars and acid recovery are critical operations whose cost has prohibited widespread use of concentrated acid. The concentrated sulfuric acid process has been commercialized in the past, particularly in the former Soviet Union, Germany, and Japan during wartime. Dilute acid hydrolysis occurs in two stages to maximize sugar yields from the hemicellulose and cellulose fractions of biomass. The first stage is operated under milder conditions to hydrolyze hemicellulose, while the second stage is optimized to hydrolyze the more resistant cellulose fraction. Liquid hydrolyzates are recovered from each stage, pH neutralized, and fermented to ethanol. However, these processes were only successful during times of national crisis, when economic competitiveness of ethanol production could be ignored.
In biomass gasification and fermentation, biomass is converted to a synthesis gas, which consists primarily of carbon monoxide, carbon dioxide, and hydrogen via a high temperature gasification process. Anaerobic bacteria are then used to convert the synthesis gas into ethanol. A practical combination of mechanico-chemical treatments and enzymes has not been commercialized, although some highly subsidized operations are being funded by the U.S. government with some private capital.
Biomass structures are naturally resistant to penetration by low levels of chemicals and/or process heat transfer, or to enzymatic hydrolysis, thus requiring high and uneconomical levels of those inputs to achieve high levels and fast rates of hydrolysis, and even with high levels of enzymes, high percentage hydrolysis is still elusive due to biomass resistance. Typically, when enzymes are used in downstream, lower temperature stages, product output is typically of low concentration and slow rates compared to that for starch hydrolysis or fermentation of sugars extracted from sugarcane, due to biological limitations of enzymes, thus increasing overall process costs and typically extending process times significantly. Methods which convert emerging sugars to ethanol, known as simultaneous saccharification/fermentation (SSF) have been under development for about 25 years with up to 2 billion dollars having been spent through the National Renewable Energy Laboratories, but has not yet proven to be a commercial process. Rates of SSF are notoriously slow, thus increasing all related costs.
Concentrated acid, dilute, high-temperature acid combinations, steam, moderate temperature/neutral pH, dry grinding, strong alkali, liquid anhydrous ammonia, high water ratios of lime, conically-shaped rotor-stator tools, a laboratory sonicating device, liquid stream, high-shear, and cavitating devices have been used to attempt to refine biomass economically. But there have been no developments to date that enable such processes to be scaled-up for larger production. There are no unsubsidized or stand-alone economical industrial-scale processes for converting high percentages of native, non-starch biomass, cellulosic portions into glucose, xylose, and downstream products made from those including organic acids or ethanol, ethyl acetate or rumen animal feed, with one exception being a small volume extracted from paper pulping, used for adhesive production. There are few industrial processes that can cost effectively dissolve biomass to produce adhesives or bioplastics to compete with petroleum based feedstock. The method currently being utilized to produce chemical precursors from biomass for adhesives or bioplastics are achieved by extracting oligomers and monomers of glucose, xylose, arabinose, galactose and other trace sugars from the paper pulp industry as “black liquor”, as well as protein and amino acids. Black liquor methods require a refining step to remove problem compounds.